1. Field of the Invention
The present invention relates to a photographic element, more particularly, to a photographic element comprising a support with a hydrophobic surface provided with one or more photographic hydrophilic colloid layers.
2. Description of the Prior Art
As photographic supports, polyethylene terephthalate (hereinafter referred to as PET) film, cellulose triacetate film, polystyrene film, polycarbonate film, polyolefin film, laminated paper, and the like have been widely used due to their superior transparency, flexibility, or the like. The use of these films or paper as photographic supports, however, is subject to the difficulty that photographic layers, the major component of which is hydrophilic gelatin, are difficult to adhere thereto because of the supports hydrophobicity.
Two types of surface treatments are known in the art to overcome such problems. In the first method, a hydrophobic support is subjected to a surface activation treatment, such as a chemical, mechanical, corona discharge, flame, ultraviolet, high frequency wave, glow discharge, activated plasma, laser, mixed acid, or ozone oxidation treatment, and then coated directly with photographic emulsion layers. In the second method, a hydrophobic support is subjected to a surface treatment as above, provided with one or more subbing layers, and then coated with photographic emulsion layers. (For example, see U.S. Pat. Nos. 2,698,241, 2,764,520, 2,864,755, 2,864,756, 2,972,534, 3,057,792, 3,071,466, 3,072,483, 3,143,421, 3,145,105, 3,145,242, 3,360,448, 3,376,208, 3,462,335 and 3,475,193; British Pat. Nos. 788,365 and 891,469, and the like.)
Of these two methods, the latter is more effective and has been more widely practiced. The increase in adhesive strength by these surface treatments may be attributable to the formation of polar groups to a greater or lesser degree on the surface of the support which is inherently hydrophobic, to the removal of a very thin surface layer which may constitute a negative factor to its adhesive strength, and to the increase in the density of cross-linking, each of which results in an increase in the affinity between the support and polar groups contained in a subbing liquid.
Various subbing methods have also been used, either by applying to the support a first layer which is adhesive thereto and then applying thereon a second layer of a hydrophilic layer (multilayer method), or by applying to the support a layer consisting of a resin containing both hydrophilic and hydrophobic groups (single layer method).
Each of these methods has been well investigated. For example, a number of resins have been investigated such as copolymers obtained from monomers selected from vinyl chloride, vinylidene chloride, methacrylic acid, acrylic acid, itaconic acid, maleic anhydride and the like, as well as polyethyleneimine, epoxy resin, grafted gelatin, nitrocellulose, and the like. However, known methods have suffered from disadvantages such as those set forth below.
(A) Adhesive strength is insufficient
In particular, in the case of hydrophobic supports such as PET and polystyrene, adhesive strength sufficient for a photographic element could not be obtained by the aforementioned surface treatment alone. In addition, if the support was not subjected to an appropriate after treatment, such as washing, there may be caused a deterioration in photographic properties, particularly a significant increase in fogging with the passage of time due to the oxidative decomposition of the surface of the support. Even in cases where a subbing layer is applied thereon, a resin which has affinity for a hydrophobic support and, therefore, is adhesive thereto, generally has a poor affinity for a hydrophilic photographic layer coated thereon, so that sufficient adhesive strength is not be obtained. On the contrary, when a hydrophilic resin having a strong affinity for a hydrophilic photographic layer is subbed, the subbing layer adheres well thereto, but only insufficiently to a hydrophobic support.
(B) A surface treatment is required
Surface treatments of a hydrophilic support, which are conducted to render its surface hydrophilic, suffer from various difficulties as described below.
Chemical treatments have the problem of variations of composition of an oxidant containing solution with the passage of time, and require surface treatments such as washing and drying. In the case of a surface treatment using a solvent which swells or dissolves the support, the flatness of the support may be deteriorated by the treatment as described hereinafter. In this case, if drying is carried out under conditions in which its flatness can be maintained, the swelling agent may remain in the support, whereby fogging may be induced in photographic emulsions.
UV treatment hardly contributes to increase the adhesiveness of a film support if it is carried out at ordinary temperature. In order to obtain good adhesion by this method, irradiation with UV must be at a temperature higher than the glass transition point of the support. In this case, the flatness of the support is apt to be impaired by a decrease in elasticity, shrinkage caused by heating, and nonuniformity of heating due to the differences in thickness. In such a case, other problems such as wrinkling, etc., may also be caused. From the viewpoint of equipment, there are disadvantages in that UV lamps have only a short life and are very expensive.
In corona discharge, static charges generated by corona discharge in the atmosphere accumulate on the surface of a PET film support, are injected into the film, and induce a leakage current, so that a very thin film of low molecular weight polymer deteriorated by oxidation is formed on the surface thereof; this is because the polar groups in PET having a very large dipole moment and act as a trap center for the transfer of the accumulated charge. By this treatment, however, sufficient adhesive strength is not obtained for a photographic layer which requires a strong stripping resistance, although fairly good results are obtained in printing properties. Accordingly, it is necessary to heat the PET film support to a temperature higher than its glass transition point. In this instance, there may be caused undesirable results similar to those with the UV treatment.
As noted above, there are also various problems in other surface treatment methods.
A surface treatment is nothing but an oxidative decomposition. By this treatment not only is the appearance of the PET film impaired, but also fog is generated due to the formation of trace amounts of oligomers, acetaldehyde, and the like. Such defects can be fatal in the photographic industry.
As will be understood from the lowering of the limiting viscosity of PET subjected to a surface treatment, the molecular weight of PET is reduced by the treatment, which becomes a problem in reusing the PET. If increased adhesion could be obtained without any surface treatment such would be preferred to make it possible to reuse the PET, which is a relatively expensive synthetic polymer, and to avoid problems relating to the disposal thereof.
(C) There are such problems as the harmful influence on human bodies', pollution, etc., due to the poisonous gases generated from the organic compounds used.
In order to further improve the adhesive strength of the aforementioned subbing layers, it is also known to incorporate into a subbing solution a large amount of a solvent having a high polarity and a high boiling point which is capable of swelling or dissolving the support, i.e., an etching agent, which is generally expensive. As etching agents for PET there are known, for example, aromatic compounds (such as those having a benzene ring, a naphthalene ring, a pyridine ring, a pyrrole ring, or a condensed ring, including those substituted by one or more groups such as alkyl, alkoxyl, acyl, nitro, cyano, halogen, hydroxyl, formyl, carboxyl, alkoxyl, carbonyl, hydroxyalkyl, aminoalkyl, haloalkyl, etc.), alcohols, ketones, carboxylic acids, esters, aldehydes, and the like. See, for example, British Pat. Nos. 772,600, 776,157, 785,789 and 797,425; U.S. Pat. No. 2,830,030; and West German Patent Nos. 1,020,457 and 1,092,652. Specific examples of etching agents include benzoic acid, salicylic acid, salicylates, monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, trifluoroacetic acid, methanesulfonic acid, nitropropanol, benzyl alcohol, benzaldehyde, acetylacetone, acetylphenol, benzamide, benzonitrile, anisole, nitrobenzyl alcohol, chlorobenzyl alcohol, pyrrole, chloral hydrate, benzylamine, xylenediamine, nicotinic acid amide and nicotinic acid esters, as well as those having at least one phenolic hydroxyl group, such as phenol, o-chlorophenol, p-chlorophenol, dichlorophenol, phenylphenol, chlororesorcin, orcinol, pyrogallol, gallic acid, o-cresol, m-cresol, p-cresol, resorcin, methoxyphenol, and the like. It is well known, however, that these etching agents are difficult to handle and are harmful to humans. Phenol compounds, in particular, have a very strong irritation and penetration effect on the skin, and hence cause many difficulties in use. In addition, full recovery of etching agents is necessary since they cause environmental pollution if contained in exhaust air or waste water. Facilities are thus often required for their recovery.
(D) The flatness of films is impaired
According to the prior art, drying of a subbing layer must be carried out at a fairly high temperature as mentioned above. In particular, a long period of heating is required for drying a subbing layer containing phenol compounds, which generally have a boiling point higher than 180.degree. C. Therefore, expansion and lowering in elasticity of the support are caused at the early stage of drying because of the increase in crystallinity based on the penetration of a solvent, and a contraction of the support is caused at the latter stage of the drying, which results in a serious impairing of its flatness.
(E) Temperature control is difficult during the drying of a subbing layer
In general, the etching power of a solvent largely depends on the temperature. Consequently, in cases where the adhesion of the subbing layer is attained by the anchoring effect of a resin, the adhesion itself largely depends on the temperature during such steps as coating, drying, etc. This is a serious problem from the viewpoint of stable production because of the difficulty of temperature control.